Method and apparatus for cabac-based entropy coding

ABSTRACT

A picture decoding method performed by a decoding device according to an embodiment of the present disclosure comprises the steps of: deriving a context index for a split flag syntax element on the basis of split availability information of a current block; determining a context model on the basis of the derived context index; decoding a value of the split flag syntax element on the basis of a CABAC, using the determined context model; deriving a current coding unit from the current block on the basis of the value of the split flag syntax element; deriving a predicted block on the basis of inter prediction or intra prediction for the current coding unit; and generating a reconstructed block on the basis of the predicted block.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119(e), this application is a continuation ofInternational Application PCT/KR2019/007213, with an internationalfiling date of Jun. 14, 2019, which claims the benefit of Korean PatentApplication No. 10-2018-0069073 filed on Jun. 15, 2018, and U.S.Provisional Application No. 62/692,887 filed on Jul. 2, 2018, thecontents of which are all hereby incorporated by reference herein intheir entirety.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The present disclosure relates to a still image or a video imageencoding/decoding method, and more specifically, to a method and anapparatus for splitting a region into a quad tree (QT) structure andusing a context model when performing CABAC entropy coding based onbinary tree (BT) and ternary tree (TT) structures.

Related Art

Demands for high-resolution and high-quality images, such as HighDefinition (HD) images and Ultra High Definition (UHD) images, areincreasing in various fields. As image data has high resolution and highquality, the amount of information or bits to be transmitted increasesrelative to legacy image data. Accordingly, when image data istransmitted using a medium, such as a conventional wired/wirelessbroadband line, or image data is stored using an existing storagemedium, a transmission cost and a storage cost thereof are increased.

Accordingly, there is a need for a highly efficient image compressiontechnique for effectively transmitting, storing, and reproducinginformation of high resolution and high quality images.

SUMMARY

An object of the present disclosure is to provide a method and anapparatus for enhancing image coding efficiency.

Another object of the present disclosure is to provide a method and anapparatus for enhancing efficiency of an image coding based on CABAC.

Still another object of the present disclosure is to provide a methodand an apparatus for determining a block structure and a block splitbased on a quad tree (QT) structure, a binary tree (BT) structure, and aternary tree (TT) structure in a process of compressing an image.

Yet another object of the present disclosure is to provide a method andan apparatus for encoding and decoding an image more efficiently using acontext model, when performing an entropy encoding according to CABACbased on the QT structure, the BT structure, and the TT structure.

An exemplary embodiment of the present disclosure provides a picturedecoding method performed by a decoding apparatus. The method includes:deriving a context index for a split flag syntax element based on splitavailability information of a current block, determining a context modelbased on the derived context index, decoding a value of the split flagsyntax element based on context-based adaptive binary arithmetic coding(CABAC) by using the determined context model, deriving a current codingunit from the current block based on the value of the split flag syntaxelement, deriving a predicted block based on inter-prediction orintra-prediction for the current coding unit, and generating areconstructed block based on the predicted block, in which the splitflag syntax element includes: a directional split flag representingwhether the current block is to be vertically split, the splitavailability information includes: an information binary tree (BT)horizontal split available flag information representing whether thecurrent block may be horizontally split based on a BT structure, aternary tree (TT) horizontal split available flag informationrepresenting whether the current block is horizontally split based on aTT structure, a BT vertical split available flag informationrepresenting whether the current block is vertically split based on theBT structure, and a TT vertical split available flag informationrepresenting whether the current block is vertically split based on theTT structure, and the context index for the directional split flag isderived based on the BT horizontal split available flag information, theTT horizontal split available flag information, the BT vertical splitavailable flag information, and the TT vertical split available flaginformation.

Another exemplary embodiment of the present disclosure provides adecoding apparatus for performing picture decoding. The decodingapparatus includes: an entropy decoder configured to derive a contextindex for a split flag syntax element based on split availabilityinformation of a current block, determine a context model based on thederived context index, decode a value of the split flag syntax elementbased on CABAC using the determined context model, and derive a currentcoding unit from the current block based on the value of the split flagsyntax element, a predictor configured to derive a predicted block basedon inter-prediction or intra-prediction for the current coding unit, andan adder configured to generate a reconstructed block based on thepredicted block, in which the split flag syntax element includes: adirectional split flag representing whether the current block isvertically split, the split availability information includes: a BThorizontal split available flag information representing whether thecurrent block may be horizontally split based on a BT structure, a TThorizontal split available flag information representing whether thecurrent block may be horizontally split based on a TT structure, a BTvertical split available flag information representing whether thecurrent block may be vertically split based on the BT structure, and aTT vertical split available flag information representing whether thecurrent block may be vertically split based on the TT structure, and thecontext index for the directional split flag is derived based on the BThorizontal split available flag information, the TT horizontal splitavailable flag information, the BT vertical split available flaginformation, and the TT vertical split available flag information.

Still another exemplary embodiment of the present disclosure provides apicture encoding method performed by an encoding apparatus. The methodincludes: deriving a split flag syntax element representing a splitstructure of a current block, deriving a context index for the splitflag syntax element based on split availability information of thecurrent block, determining a context model based on the derived contextindex, and encoding a value of the split flag syntax element based onCABAC using the determined context model, in which the split flag syntaxelement includes: a directional split flag representing whether thecurrent block is to be vertically split, the split availabilityinformation includes: a BT horizontal split available flag informationrepresenting whether the current block may be horizontally split basedon a BT structure, a TT horizontal split available flag informationrepresenting whether the current block may be horizontally split basedon a TT structure, a BT vertical split available flag informationrepresenting whether the current block may be vertically split based onthe BT structure, and a TT vertical split available flag informationrepresenting whether the current block may be vertically split based onthe TT structure, and the context index for the directional split flagis derived based on the BT horizontal split available flag information,the TT horizontal split available flag information, the BT verticalsplit available flag information, and the TT vertical split availableflag information.

Yet another exemplary embodiment of the present disclosure provides anencoding apparatus for performing picture encoding. The encodingapparatus includes: an entropy encoder configured to derive a split flagsyntax element representing a split structure of a current block, derivea context index for the split flag syntax element based on splitavailability information of the current block, determine a context modelbased on the derived context index, and encode a value of the split flagsyntax element based on CABAC using the determined context model, inwhich the split flag syntax element includes: a directional split flagrepresenting whether the current block is to be vertically split, thesplit availability information includes: a BT horizontal split availableflag information representing whether the current block may behorizontally split based on a BT structure, a TT horizontal splitavailable flag information representing whether the current block may behorizontally split based on a TT structure, a BT vertical splitavailable flag information representing whether the current block may bevertically split based on the BT structure, and a TT vertical splitavailable flag information representing whether the current block may bevertically split based on the TT structure, and the context index forthe directional split flag is derived based on the BT horizontal splitavailable flag information, the TT horizontal split available flaginformation, the BT vertical split available flag information, and theTT vertical split available flag information.

The present disclosure may enhance the overall image/video compressionefficiency.

The present disclosure may enhance the efficiency of the image codingbased on the intra-prediction.

The present disclosure may enhance the efficiency of the image codingbased on the CABAC.

The present disclosure may improve the pipeline delay, when implementingthe intra-prediction based on the CCLM in hardware.

The present disclosure may determine the block structure and the blocksplit based on the quad tree (QT) structure, the binary tree (BT)structure, and the ternary tree (TT) structure in the process ofcompressing the image.

The present disclosure may encode and decode the image more efficientlyusing the context model, when performing the entropy encoding accordingto the CABAC based on the QT structure, the BT structure, and the TTstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for schematically explaining a configuration of anencoding apparatus according to an exemplary embodiment.

FIG. 2 is a diagram for schematically explaining a configuration of adecoding apparatus according to the exemplary embodiment.

FIGS. 3A to 3C are diagrams illustrating examples of a QT split, a BTsplit, and a TT split.

FIG. 4 is a block diagram illustrating a configuration of an entropyencoder according to the exemplary embodiment.

FIG. 5 is a block diagram illustrating a configuration of an entropydecoder according to the exemplary embodiment.

FIG. 6 is a flowchart illustrating an operation of the entropy encoderaccording to the exemplary embodiment.

FIG. 7 is a flowchart illustrating an operation of the entropy decoderaccording to the exemplary embodiment.

FIG. 8 is a flowchart illustrating an operation of the encodingapparatus according to the exemplary embodiment.

FIG. 9 is a block diagram illustrating a configuration of the encodingapparatus according to the exemplary embodiment.

FIG. 10 is a flowchart illustrating an operation of the decodingapparatus according to the exemplary embodiment.

FIG. 11 is a block diagram illustrating a configuration of the decodingapparatus according to the exemplary embodiment.

FIG. 12 is a contents streaming system structure diagram according tothe exemplary embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

An exemplary embodiment of the present disclosure provides a picturedecoding method performed by a decoding apparatus. The method includes:deriving a context index for a split flag syntax element based on spitavailability information of a current block, determining a context modelbased on the derived context index, decoding a value of the split flagsyntax element based on context-based adaptive binary arithmetic coding(CABAC) using the determined context model, deriving a current codingunit from the current block based on the value of the split flag syntaxelement, deriving a predicted block based on inter-prediction orintra-prediction for the current coding unit, and generating areconstructed block based on the predicted block, in which the splitflag syntax element includes a directional split flag representingwhether the current block is vertically split; the split availabilityinformation includes BT horizontal split available flag informationrepresenting whether the current block may be horizontally split basedon a binary tree (BT) structure, TT horizontal split available flaginformation representing whether the current block may be horizontallysplit based on the ternary tree (TT) structure, BT vertical splitavailable flag information representing whether the current block may bevertically split based on the BT structure, and TT vertical splitavailable flag information representing whether the current block may bevertically split based on the TT structure; and the context index forthe directional split flag is derived based on the BT horizontal splitavailable flag information, the TT horizontal split available flaginformation, the BT vertical split available flag information, and theTT vertical split available flag information.

Since the present disclosure may be variously modified and may havevarious exemplary embodiments, specific exemplary embodiments will beillustrated in the drawings and described in detail. However, theembodiments are not intended for limiting the disclosure. The terms usedin the following description are used to merely describe specificembodiments, but are not intended to limit the disclosure. An expressionof a singular number includes an expression of the plural number, solong as it is clearly read differently. The terms such as “include” and“have” are intended to indicate that features, numbers, steps,operations, elements, components, or combinations thereof used in thefollowing description exist and it should be thus understood that thepossibility of existence or addition of one or more different features,numbers, steps, operations, elements, components, or combinationsthereof is not excluded.

Meanwhile, elements in the drawings described in the disclosure areindependently drawn for the purpose of convenience for explanation ofdifferent specific functions, and do not mean that the elements areembodied by independent hardware or independent software. For example,two or more elements of the elements may be combined to form a singleelement, or one element may be divided into plural elements. Theembodiments in which the elements are combined and/or divided belong tothe disclosure without departing from the concept of the disclosure.

The following description relates to a video/image coding. For example,the method/exemplary embodiments disclosed in this document may beapplied to a method disclosed in a versatile video coding (VVC)standard, an essential video coding (EVC) standard, an AOMedia Video 1(AV1) standard, a 2nd generation of audio video coding (AVS2) standard,or a next-generation video/image coding standard (e.g., H.267, H.268, orthe like).

Hereinafter, examples of the present embodiment will be described indetail with reference to the accompanying drawings. In addition, likereference numerals are used to indicate like elements throughout thedrawings, and the same descriptions on the like elements will beomitted.

In the present disclosure, a video may mean a set of a series of imagesaccording to a passage of time. Generally, a picture means a unitrepresenting an image at a specific time, a slice is a unit constitutinga part of the picture. One picture may be composed of plural slices, andthe terms of a picture and a slice may be mixed with each other asoccasion demands.

A pixel or a pel may mean a minimum unit constituting one picture (orimage). Further, a “sample” may be used as a term corresponding to apixel. The sample may generally represent a pixel or a value of a pixel,may represent only a pixel (a pixel value) of a luma component, and mayrepresent only a pixel (a pixel value) of a chroma component.

A unit indicates a basic unit of image processing. The unit may includeat least one of a specific area and information about the area.Optionally, the unit may be mixed with terms such as a block, an area,or the like. In a typical case, an M×N block may represent a set ofsamples or transform coefficients arranged in M columns and N rows.

FIG. 1 is a diagram briefly illustrating a structure of an encodingapparatus to which the present disclosure is applicable. Hereinafter, anencoding/decoding apparatus may include a video encoding/decodingapparatus and/or an image encoding/decoding apparatus, and the videoencoding/decoding apparatus may be used as a concept comprising theimage encoding/decoding apparatus, or the image encoding/decodingapparatus may be used as a concept comprising the videoencoding/decoding apparatus.

Referring to FIG. 1, a video encoding apparatus 100 may include apicture partitioner 105, a predictor 110, a residual processer 120, anentropy encoder 130, an adder 140, a filter 150, and a memory 160. Theresidual processer 120 may include a subtractor 121, a transformer 122,a quantizer 123, a re-arranger 124, a dequantizer 125, an inversetransformer 126.

The picture partitioner 105 may split an input picture into at least oneprocessing unit.

In an example, the processing unit may be referred to as a coding unit(CU). In this case, the coding unit may be recursively split from thelargest coding unit (LCU) according to a quad-tree binary-tree (QTBT)structure. For example, one coding unit may be split into a plurality ofcoding units of a deeper depth based on a quadtree structure, a binarytree structure and/or a ternary tree structure. In this case, forexample, the quad tree structure may be first applied and the binarytree structure and ternary tree structure may be applied later.Alternatively, the binary tree structure/ternary tree structure may beapplied first. The coding procedure according to the present embodimentmay be performed based on a final coding unit which is not split anyfurther. In this case, the largest coding unit may be used as the finalcoding unit based on coding efficiency, or the like, depending on imagecharacteristics, or the coding unit may be recursively split into codingunits of a lower depth as necessary and a coding unit having an optimalsize may be used as the final coding unit. Here, the coding proceduremay include a procedure such as prediction, transform, andreconstruction, which will be described later.

In another example, the processing unit may include a coding unit (CU)prediction unit (PU), or a transformer (TU). The coding unit may besplit from the largest coding unit (LCU) into coding units of a deeperdepth according to the quad tree structure. In this case, the largestcoding unit may be directly used as the final coding unit based on thecoding efficiency, or the like, depending on the image characteristics,or the coding unit may be recursively split into coding units of adeeper depth as necessary and a coding unit having an optimal size maybe used as a final coding unit. When the smallest coding unit (SCU) isset, the coding unit may not be split into coding units less than thesmallest coding unit. Here, the final coding unit refers to a codingunit which is partitioned or split to a prediction unit or atransformer. The prediction unit is a unit which is partitioned from acoding unit, and may be a unit of sample prediction. Here, theprediction unit may be divided into sub-blocks. The transformer may bedivided from the coding unit according to the quad-tree structure andmay be a unit for deriving a transform coefficient and/or a unit forderiving a residual signal from the transform coefficient. Hereinafter,the coding unit may be referred to as a coding block (CB), theprediction unit may be referred to as a prediction block (PB), and thetransformer may be referred to as a transform block (TB). The predictionblock or prediction unit may refer to a specific area in the form of ablock in a picture and include an array of prediction samples. Also, thetransform block or transformer may refer to a specific area in the formof a block in a picture and include the transform coefficient or anarray of residual samples.

The predictor 110 may perform prediction on a processing target block(hereinafter, it may represent a current block or a residual block), andmay generate a predicted block including prediction samples for thecurrent block. A unit of prediction performed in the predictor 110 maybe a coding block, or may be a transform block, or may be a predictionblock.

The predictor 110 may determine whether intra-prediction is applied orinter-prediction is applied to the current block. For example, thepredictor 110 may determine whether the intra-prediction or theinter-prediction is applied in unit of CU.

In case of the intra-prediction, the predictor 110 may derive aprediction sample for the current block based on a reference sampleoutside the current block in a picture to which the current blockbelongs (hereinafter, a current picture). In this case, the predictor110 may derive the prediction sample based on an average orinterpolation of neighboring reference samples of the current block(case (I)), or may derive the prediction sample based on a referencesample existing in a specific (prediction) direction as to a predictionsample among the neighboring reference samples of the current block(case (ii)). The case (a) may be called a non-directional mode or anon-angular mode, and the case (ii) may be called a directional mode oran angular mode. In the intra-prediction, prediction modes may includeas an example 33 directional modes and at least two non-directionalmodes. The non-directional modes may include DC mode and planar mode.The predictor 110 may determine the prediction mode to be applied to thecurrent block by using the prediction mode applied to the neighboringblock.

In case of the inter-prediction, the predictor 110 may derive theprediction sample for the current block based on a sample specified by amotion vector on a reference picture. The predictor 110 may derive theprediction sample for the current block by applying any one of a skipmode, a merge mode, and a motion vector prediction (MVP) mode. In caseof the skip mode and the merge mode, the predictor 110 may use motioninformation of the neighboring block as motion information of thecurrent block. In case of the skip mode, unlike in the merge mode, adifference (residual) between the prediction sample and an originalsample is not transmitted. In case of the MVP mode, a motion vector ofthe neighboring block is used as a motion vector predictor to derive amotion vector of the current block.

In case of the inter-prediction, the neighboring block may include aspatial neighboring block existing in the current picture and a temporalneighboring block existing in the reference picture. The referencepicture including the temporal neighboring block may also be called acollocated picture (colPic). Motion information may include the motionvector and a reference picture index. Information such as predictionmode information and motion information may be (entropy) encoded, andthen output as a form of a bit stream.

When motion information of a temporal neighboring block is used in theskip mode and the merge mode, the highest picture in a reference picturelist may be used as a reference picture. Reference pictures included inthe reference picture list may be aligned based on a picture order count(POC) difference between a current picture and a corresponding referencepicture. A POC corresponds to a display order and may be discriminatedfrom a coding order.

The subtractor 121 generates a residual sample which is a differencebetween an original sample and a prediction sample. If the skip mode isapplied, the residual sample may not be generated as described above.

The transformer 122 transforms residual samples in units of a transformblock to generate a transform coefficient. The transformer 122 mayperform transform based on the size of a corresponding transform blockand a prediction mode applied to a coding block or prediction blockspatially overlapping with the transform block. For example, residualsamples may be transformed using discrete sine transform (DST) transformkernel if intra-prediction is applied to the coding block or theprediction block overlapping with the transform block and the transformblock is a 4×4 residual array and is transformed using discrete cosinetransform (DCT) transform kernel in other cases.

The quantizer 123 may quantize the transform coefficients to generatequantized transform coefficients.

The re-arranger 124 rearranges quantized transform coefficients. There-arranger 124 may rearrange the quantized transform coefficients inthe form of a block into a one-dimensional vector through a coefficientscanning method. Although the re-arranger 124 is described as a separatecomponent, the re-arranger 124 may be a part of the quantizer 123.

The entropy encoder 130 may perform entropy-encoding on the quantizedtransform coefficients. The entropy encoding may include an encodingmethod, for example, an exponential Golomb, a context-adaptive variablelength coding (CAVLC), a context-adaptive binary arithmetic coding(CABAC), or the like. The entropy encoder 130 may perform encoding,according to an entropy encoding or according to a pre-configuredmethod, together or separately on information (e.g., a syntax elementvalue or the like) required for video reconstruction in addition to thequantized transform coefficients. The entropy-encoded information may betransmitted or stored in unit of a network abstraction layer (NAL) in abit stream form. The bitstream may be transmitted via a network or bestored in a digital storage medium. Here, the network may include abroadcasting network or a communications network, the digital storagemedium may include various storage medium such as USB, SD, CD, DVD,blue-ray, HDD, SDD and so on.

The dequantizer 125 dequantizes values (transform coefficients)quantized by the quantizer 123 and the inverse transformer 126 inverselytransforms values dequantized by the dequantizer 125 to generate aresidual sample.

The adder 140 adds a residual sample to a prediction sample toreconstruct a picture. The residual sample may be added to theprediction sample in units of a block to generate a reconstructed block.Although the adder 140 is described as a separate component, the adder140 may be a part of the predictor 110. Meanwhile, the adder 140 may bereferred to as a reconstructor or reconstructed block generator.

The filter 150 may apply deblocking filtering and/or a sample adaptiveoffset to the reconstructed picture. Artifacts at a block boundary inthe reconstructed picture or distortion in quantization may be correctedthrough deblocking filtering and/or sample adaptive offset. Sampleadaptive offset may be applied in units of a sample after deblockingfiltering is completed. The filter 150 may apply an adaptive loop filter(ALF) to the reconstructed picture. The ALF may be applied to thereconstructed picture to which deblocking filtering and/or sampleadaptive offset has been applied.

The memory 160 may store a reconstructed picture (decoded picture) orinformation necessary for encoding/decoding. Here, the reconstructedpicture may be the reconstructed picture filtered by the filter 150. Thestored reconstructed picture may be used as a reference picture for(inter) prediction of other pictures. For example, the memory 160 maystore (reference) pictures used for inter-prediction. Here, picturesused for inter-prediction may be designated according to a referencepicture set or a reference picture list.

FIG. 2 is a diagram briefly illustrating a structure of a video/imagedecoding apparatus to which the present disclosure is applicable.Hereinafter, a video decoding apparatus may include an image decodingapparatus.

Referring to FIG. 2, a video decoding apparatus 200 may include anentropy decoder 210, a residual processer 220, a predictor 230, an adder240, a filter 250, and a memory 260. The residual processer 220 mayinclude a re-arranger 221, a dequantizer 222, an inverse transformer223. Further, although it is not depicted, the video decoding apparatus200 may include a receiver for receiving a bitstream including videoinformation. The receiver may be configured as a separate module or maybe included in the entropy decoder 210.

When a bit stream including video/image information is input, the videodecoding apparatus 200 may reconstruct a video/image/picture inassociation with a process by which video information is processed inthe video encoding apparatus.

For example, the video decoding apparatus 200 may perform video decodingusing a processing unit applied in the video encoding apparatus. Thus,the processing unit block of video decoding may be, for example, acoding unit and, in another example, a coding unit, a prediction unit ora transformer. The coding unit may be split from the largest coding unitaccording to the quad tree structure and/or the binary tree structureand/or ternary tree structure.

A prediction unit and a transformer may be further used in some cases,and in this case, the prediction block is a block derived or partitionedfrom the coding unit and may be a unit of sample prediction. Here, theprediction unit may be divided into sub-blocks. The transformer may besplit from the coding unit according to the quad tree structure and maybe a unit that derives a transform coefficient or a unit that derives aresidual signal from the transform coefficient.

The entropy decoder 210 may parse the bit stream to output informationrequired for video reconstruction or picture reconstruction. Forexample, the entropy decoder 210 may decode information in the bitstream based on a coding method such as exponential Golomb encoding,CAVLC, CABAC, or the like, and may output a value of a syntax elementrequired for video reconstruction and a quantized value of a transformcoefficient regarding a residual.

More specifically, a CABAC entropy decoding method may receive a bincorresponding to each syntax element in a bit stream, determine acontext model using decoding target syntax element information anddecoding information of neighboring and decoding target blocks orinformation of symbol/bin decoded in a previous step, predict bingeneration probability according to the determined context model andperform arithmetic decoding of the bin to generate a symbolcorresponding to each syntax element value. Here, the CABAC entropydecoding method may update the context model using information of asymbol/bin decoded for a context model of the next symbol/bin afterdetermination of the context model.

Information about prediction among information decoded in the entropydecoder 210 may be provided to the predictor 250 and residual values,that is, quantized transform coefficients, on which entropy decoding hasbeen performed by the entropy decoder 210 may be input to there-arranger 221.

The re-arranger 221 may rearrange the quantized transform coefficientsinto a two-dimensional block form. The re-arranger 221 may performrearrangement corresponding to coefficient scanning performed by theencoding apparatus. Although the re-arranger 221 is described as aseparate component, the re-arranger 221 may be a part of the dequantizer222.

The dequantizer 222 may de-quantize the quantized transform coefficientsbased on a (de)quantization parameter to output a transform coefficient.In this case, information for deriving a quantization parameter may besignaled from the encoding apparatus.

The inverse transformer 223 may inverse-transform the transformcoefficients to derive residual samples.

The predictor 230 may perform prediction on a current block, and maygenerate a predicted block including prediction samples for the currentblock. A unit of prediction performed in the predictor 230 may be acoding block or may be a transform block or may be a prediction block.

The predictor 230 may determine whether to apply intra-prediction orinter-prediction based on information on a prediction. In this case, aunit for determining which one will be used between the intra-predictionand the inter-prediction may be different from a unit for generating aprediction sample. In addition, a unit for generating the predictionsample may also be different in the inter-prediction and theintra-prediction. For example, which one will be applied between theinter-prediction and the intra-prediction may be determined in unit ofCU. Further, for example, in the inter-prediction, the prediction samplemay be generated by determining the prediction mode in unit of PU, andin the intra-prediction, the prediction sample may be generated in unitof TU by determining the prediction mode in unit of PU.

In case of the intra-prediction, the predictor 230 may derive aprediction sample for a current block based on a neighboring referencesample in a current picture. The predictor 230 may derive the predictionsample for the current block by applying a directional mode or anon-directional mode based on the neighboring reference sample of thecurrent block. In this case, a prediction mode to be applied to thecurrent block may be determined by using an intra-prediction mode of aneighboring block.

In the case of inter-prediction, the predictor 230 may derive aprediction sample for a current block based on a sample specified in areference picture according to a motion vector. The predictor 230 mayderive the prediction sample for the current block using one of the skipmode, the merge mode, and the MVP mode. Here, motion informationrequired for inter-prediction of the current block provided by the videoencoding apparatus, for example, a motion vector and information about areference picture index may be acquired or derived based on theinformation about prediction.

In the skip mode and the merge mode, motion information of a neighboringblock may be used as motion information of the current block. Here, theneighboring block may include a spatial neighboring block and a temporalneighboring block.

The predictor 230 may construct a merge candidate list using motioninformation of available neighboring blocks and use informationindicated by a merge index on the merge candidate list as a motionvector of the current block. The merge index may be signaled by theencoding apparatus. Motion information may include a motion vector and areference picture. In the skip mode and the merge mode, afirstly-ordered picture in the reference picture list may be used as areference picture when motion information of a temporal neighboringblock is used.

In the case of the skip mode, a difference (residual) between aprediction sample and an original sample is not transmitted,distinguished from the merge mode.

In the case of the MVP mode, the motion vector of the current block maybe derived using a motion vector of a neighboring block as a motionvector predictor. Here, the neighboring block may include a spatialneighboring block and a temporal neighboring block.

When the merge mode is applied, for example, a merge candidate list maybe generated using a motion vector of a reconstructed spatialneighboring block and/or a motion vector corresponding to a Col blockwhich is a temporal neighboring block. A motion vector of a candidateblock selected from the merge candidate list is used as the motionvector of the current block in the merge mode. The aforementionedinformation about prediction may include a merge index indicating acandidate block having the best motion vector selected from candidateblocks included in the merge candidate list. Here, the predictor 230 mayderive the motion vector of the current block using the merge index.

When the MVP (Motion vector Prediction) mode is applied as anotherexample, a motion vector predictor candidate list may be generated usinga motion vector of a reconstructed spatial neighboring block and/or amotion vector corresponding to a Col block which is a temporalneighboring block. That is, the motion vector of the reconstructedspatial neighboring block and/or the motion vector corresponding to theCol block which is the temporal neighboring block may be used as motionvector candidates. The aforementioned information about prediction mayinclude a prediction motion vector index indicating the best motionvector selected from motion vector candidates included in the list.Here, the predictor 230 may select a prediction motion vector of thecurrent block from the motion vector candidates included in the motionvector candidate list using the motion vector index. The predictor ofthe encoding apparatus may obtain a motion vector difference (MVD)between the motion vector of the current block and a motion vectorpredictor, encode the MVD and output the encoded MVD in the form of abit stream. That is, the MVD may be obtained by subtracting the motionvector predictor from the motion vector of the current block. Here, thepredictor 230 may acquire a motion vector included in the informationabout prediction and derive the motion vector of the current block byadding the motion vector difference to the motion vector predictor. Inaddition, the predictor may obtain or derive a reference picture indexindicating a reference picture from the aforementioned information aboutprediction.

The adder 240 may add a residual sample to a prediction sample toreconstruct a current block or a current picture. The adder 240 mayreconstruct the current picture by adding the residual sample to theprediction sample in units of a block. When the skip mode is applied, aresidual is not transmitted and thus the prediction sample may become areconstructed sample. Although the adder 240 is described as a separatecomponent, the adder 240 may be a part of the predictor 230. Meanwhile,the adder 240 may be referred to as a reconstructor reconstructed blockgenerator.

The filter 250 may apply deblocking filtering, sample adaptive offsetand/or ALF to the reconstructed picture. Here, sample adaptive offsetmay be applied in units of a sample after deblocking filtering. The ALFmay be applied after deblocking filtering and/or application of sampleadaptive offset.

The memory 260 may store a reconstructed picture (decoded picture) orinformation necessary for decoding. Here, the reconstructed picture maybe the reconstructed picture filtered by the filter 250. For example,the memory 260 may store pictures used for inter-prediction. Here, thepictures used for inter-prediction may be designated according to areference picture set or a reference picture list. A reconstructedpicture may be used as a reference picture for other pictures. Thememory 260 may output reconstructed pictures in an output order.

Meanwhile, as described above, in performing video coding, prediction isperformed to improve compression efficiency. Through this, a predictedblock including prediction samples for a current block as a block to becoded (i.e., a coding target block) may be generated. Here, thepredicted block includes prediction samples in a spatial domain (orpixel domain). The predicted block is derived in the same manner in anencoding apparatus and a decoding apparatus, and the encoding apparatusmay signal information (residual information) on residual between theoriginal block and the predicted block, rather than an original samplevalue of an original block, to the decoding apparatus, therebyincreasing image coding efficiency. The decoding apparatus may derive aresidual block including residual samples based on the residualinformation, add the residual block and the predicted block to generatereconstructed blocks including reconstructed samples, and generate areconstructed picture including the reconstructed blocks.

The residual information may be generated through a transform andquantization procedure. For example, the encoding apparatus may derive aresidual block between the original block and the predicted block,perform a transform procedure on residual samples (residual samplearray) included in the residual block to derive transform coefficients,perform a quantization procedure on the transform coefficients to derivequantized transform coefficients, and signal related residualinformation to the decoding apparatus (through a bit stream). Here, theresidual information may include value information of the quantizedtransform coefficients, position information, a transform technique, atransform kernel, a quantization parameter, and the like. The decodingapparatus may perform dequantization/inverse transform procedure basedon the residual information and derive residual samples (or residualblocks). The decoding apparatus may generate a reconstructed picturebased on the predicted block and the residual block. Also, for referencefor inter-prediction of a picture afterward, the encoding apparatus mayalso dequantize/inverse-transform the quantized transform coefficientsto derive a residual block and generate a reconstructed picture basedthereon.

FIGS. 3A to 3C are diagrams illustrating examples of a QT split, a BTsplit, and a TT split.

In a video coding, one block may be split based on a quad tree (QT).Further, one sub block split by the QT may be further split recursivelyusing the QT. A leaf block not split by the QT any more may be split bya binary tree (BT), a ternary tree (TT), a stripe tree (ST), or thelike. The BT may have two forms of splits such as a horizontal BT (2N×N,2N×N) and a vertical BT (N×2N, N×2N). The TT may have two forms ofsplits such as a horizontal TT (2N×1/2N, 2N×N, 2N×1/2N) and a verticalTT (1/2N×2N, N×2N, 1/2N×2N). The ST may have two forms of splits such asa horizontal ST (2N×1/2N, 2N×1/2N, 2N×1/2N, 2N×1/2N) and a vertical ST(1/2N×2N, 1/2N×2N, 1/2N×2N, 1/2N×2N). Each of the BT, the TT, and the STmay be further split recursively using the BT, the TT, and the ST.

FIG. 3A illustrates an example of the QT split. A block A may be splitinto four sub blocks (A0, A1, A2, A3) by the QT. The sub block A1 may besplit into four sub blocks (B0, B1, B2, B3) by the QT again.

FIG. 3B illustrates an example of the BT split. The block B3 not splitby the QT any more may be split into a vertical BT (C0, C1) or ahorizontal BT (D0, D1). Like the block C0, each sub block may be furthersplit recursively like the form of a horizontal BT (E0, E1) or avertical BT (F0, F1).

FIG. 3C illustrates an example of the TT split. The block B3 not splitby the QT any more may be split into a vertical TT (C0, C1, C2) or ahorizontal TT (D0, D1, D2). Like the block C1, each sub block may befurther split recursively like the form of a horizontal TT (E0, E1, E2)or a vertical TT (F0, F1, F2).

Table 1 below expresses an example of coding quadtree syntaxes, in thecase of using the BT and the TT.

TABLE 1 Descriptor coding_quadtree( x0, y0, log2CbSize, cqtDepth ) { if( x0 + ( 1 << log2CbSize ) <= pic_width_in_luma_samples && y0 + ( 1 <<log2CbSize ) <= pic_height_in_luma_samples && log2CbSize >MinCbLog2SizeY ) split_qt_flag[ x0 ][ y0 ] ae(v) if ( split_cu_flag[ x0][ y0 ] ) { x1 = x0 + ( 1 << ( log2CbSize − 1 ) ) y1 = y0 + ( 1 << (log2CbSize − 1 ) ) coding_quadtree( x0, y0, log2CbSize −1, cqtDepth+1 )coding_quadtree( x1, y0, log2CbSize − 1, cqtDepth + 1 ) coding_quadtree(x1, y0, log2CbSize − 1, cqtDepth + 1 ) coding_quadtree( x1, y0,log2CbSize − 1, cqtDepth + 1 ) } else { coding_tree( x0, y0, log2CbSize,log2CbSize) } }

A syntax element split_qt_flag represents whether a current (encoding)block is split into four by the QT. When a value of the split_qt_flag is0, it means that the current block is not split by the QT, and when thevalue of the split_qt_flag is 1, it means that the current block issplit by the QT. The x0 and the y0 represent a top-left location of aluma image.

The present specification uses specific terms or sentences for definingspecific information or concept. For example, the present specificationuses specific terms or sentences for defining a specific syntax element.As an example, the present specification defines the syntax elementrepresenting whether the current (encoding) block is split into 4 by theQT as the “split_qt_flag”, and defines a syntax element representingwhether the current (encoding) block is further split as a“split_fur_flag”. However, the “split_qt_flag” may be replaced withvarious terms such as a qt_split_flag and a QT split flag, and the“split_fur_flag” may be replaced with various terms such as afurther_split_flag and a fur_split_flag, such that the specific terms orsentences used for defining the specific information or concept in thepresent specification should not be interpreted as being limited tothose names throughout the specification, and need to be interpreted byfocusing on various operations, functions, and effects according to thecontents represented by the terms.

Table 2 below expresses an example of the syntaxes for the coding tree.

TABLE 2 Descriptor coding_tree( x0, y0, log2CbSizeW, log2CbSizeH) { if (x0 + ( 1 << log2CbSize ) <= pic_width_in_luma_samples && y0 + ( 1 <<log2CbSize ) <= pic_height_in_luma_samples && log2CbSize >MinBTTCbLog2SizeY ) split_fur_flag[ x0 ][ y0 ] ae(v) if (split_fur_flag[ x0 ][ y0 ] ) { split_bt_flag[ x0 ][ y0 ] ae(v)split_dir[ x0 ][ y0 ] ae(v) } if( SplitMode == PART_BT_HOR) {coding_tree ( x0, y0, log2CbSizeW, log2CbSizeH − 1 ) coding_ tree ( x0,y0 + (1 << ( log2CbSizeH − 1 )), log2CbSizeW, log2CbSizeH − 1) } else if( SplitMode == PART_BT_VER) { coding_tree ( x0, y0, log2CbSizeW − 1,log2CbSizeH) coding_tree ( x0 + (1 << ( log2CbSizeW − 1 )), y0,log2CbSizeW − 1, log2CbSizeH) } else if ( SplitMode == PART_TT_HOR) {coding_tree ( x0, y0, log2CbSizeW, log2CbSizeH − 2 ) coding_tree ( x0,y0 + (1 << ( log2CbSizeH − 2 )), log2CbSizeW, log2CbSizeH − 1)coding_tree ( x0, y0 + (1 << ( log2CbSizeH − 2 ) + (1 << ( log2CbSizeH −1 )), log2CbSizeW, log2CbSizeH − 2) } else if ( SplitMode ==PART_TT_VER) { coding_tree ( x0, y0, log2CbSizeW − 2, log2CbSizeH)coding_tree ( x0 + (1 << ( log2CbSizeW − 2 )), y0, log2CbSizeW − 1,log2CbSizeH) coding_tree ( x0 + (1 << ( log2CbSizeW − 2 ) + (1 << (log2CbSizeW − 1 )), y0, log2CbSizeW − 2, log2CbSizeH) } } else {coding_unit(x0, y0, log2CbSizeW, log2CbSizeH) } }

The syntax element split_fur_flag represents whether the current(encoding) block is further split. When a value of the split_fur_flag is0, it means that the current block is not split any more, and when thevalue of the split_fur_flag is 1, it means that the current block issplit. The syntax element split_bt_flag represents whether the currentblock is split based on the BT or the TT. When a value of thesplit_bt_flag is 1, it means that the current block is split based onthe BT, and when the value of the split_bt_flag is 0, it means that thecurrent block is split based on the TT.

A split type (SplitType) of the current block may be determined asexpressed in Table 3 below based on the syntax element split_fur_flagand the syntax element split_bt_flag.

TABLE 3 split_fur_flag split_bt_flag SplitType 0 NO_SPLIT 1 1 BT 1 0 TT

Referring to Table 3, when the value of the split_fur_flag is 0 and thecurrent block is not split any more, it may be confirmed that theSplitType is determined as NO_SPLIT. Further, when the value of thesplit_fur_flag is 1 and the current block is split, it may be confirmedthat the SplitType is determined as the BT when the value of thesplit_bt_flag is 1, and the SplitType is determined as the TT when thevalue of the split_bt_flag is 0.

The syntax element split_dir represents a direction in which the currentblock is split. When a value of the split_dir is 0, it means that thecurrent block is horizontally split, and when the value of the split_diris 1, it means that the current block is vertically split. A split mode(SplitMode) of the block may be induced as expressed in Table 4 below,based on the SplitType and the split_dir.

TABLE 4 SplitType split_dir SplitMode BT 0 PART_BT_HOR 1 PART_BT_VER TT0 PART_TT_HOR 1 PART_TT_VER

Referring to the Table 4, when the SplitType is the BT, it may beconfirmed that the SplitMode is determined as the PART_BT_HOR, that is,the BT horizontal split mode when the value of the split_dir is 0, andthe SplitMode is determined as the PART_BT_VER, that is, the BT verticalsplit mode when the value of the split_dir is 1. Further, when theSplitType is the TT, it may be confirmed that the SplitMode isdetermined as the PART_TT_HOR, that is, the TT horizontal split modewhen the value of the split_dir is 0, and the SplitMode is determined asthe PART_TT_VER, that is, the TT vertical split mode when the value ofthe split_dir is 1.

When the region of each image is split into the QT structure, theencoder and the decoder may transmit the syntax element adjusting thesplit degree of the QT in consideration of the relationship betweenperformance and complexity. For example, the syntax elements such as themaximum QT block size (MaxQTSize), the minimum QT block size(MinQTSize), and the maximum QT block split depth (MaxQTDepth) may betransmitted. Here, the maximum QT block size may mean the size of themaximum block first split by the QT, and be expressed in the form of thelog 2 scale. The minimum QT block size may mean the size of the smallestblock by the QT split, and be expressed in the form of the log 2 scale.Further, the maximum QT split depth may mean how many times QT splitsare allowed from the maximum QT block size.

When the region of each image is split into the QT structure and thenthe block is split into the BT structure, the encoder and the decodermay transmit the syntax element adjusting the split degree of the BT inconsideration of the relationship between the performance andcomplexity. For example, the syntax elements such as the maximum BTblock size (MaxBTSize), the minimum BT block size (MinBTSize), and themaximum BT block split depth (MaxBTDepth) may be transmitted. Morespecifically, the maximum BT block size may mean the size of the maximumblock in which the BT split is first allowed, and be expressed in theform of the log 2 scale. The minimum BT block size may mean the size ofthe smallest block by the BT split, and be expressed in the form of thelog 2 scale. Further, the maximum BT split depth may mean how many timesBT splits are allowed from the maximum BT block size.

When the region of each image is split into the QT structure and thenthe block is split into the TT structure, the encoder and the decodermay transmit the syntax element adjusting the split degree of the TT inconsideration of the relationship between performance and complexity.For example, the syntax elements such as the maximum TT block size(MaxTTSize), the minimum TT block size (MinTTSize), and the maximum TTblock split depth (MaxTTDepth) may be transmitted. More specifically,the maximum TT block size may mean the size of the maximum block inwhich the TT split is first allowed, and be expressed in the form of thelog 2 scale. The minimum TT block size may mean the size of the smallestblock by the TT split, and be expressed in the form of the log 2 scale.Further, the maximum TT split depth may mean how many times TT splitsare allowed from the maximum TT block size.

FIG. 4 is a block diagram illustrating a configuration of an entropyencoder according to the exemplary embodiment.

FIG. 4 illustrates a schematic block diagram of the entropy encoder towhich the context-based adaptive binary arithmetic coding (CABAC) isapplied, as an exemplary embodiment to which the present disclosure isapplied.

An entropy encoder 400 to which the present disclosure is appliedincludes a binarization unit 410, a context modeling unit 420, a binaryarithmetic encoding unit 430, and a memory 460, in which the binaryarithmetic encoding unit 430 may include a regular binary encoding unit440 and a bypass binary encoding unit 450. Here, the regular binaryencoding unit 440 and the bypass binary encoding unit 450 may also becalled a regular coding engine and a bypass coding engine, respectively.

The binarization unit 410 may receive the sequence of data symbols andperform binarization, thereby outputting a binary symbol (bin) stringcomposed of a binarized value of 0 or 1. The binarization unit 410 maymap the syntax elements with the binary symbols. Various differentbinarization processes, for example, unary (U), truncated unary (TU),kth Exp-Golomb (EGk), fixed length processes, and the like may be usedfor binarization. The binarization process may be selected based on theform of the syntax element.

The outputted bin string may be transmitted to the context modeling unit420.

The context modeling unit 420 may select probability informationnecessary for coding the current block from the memory to transmit theprobability information to the binary arithmetic encoding unit 430. Forexample, the context modeling unit 420 may select a context memory basedon a syntax element to be coded and select the probability informationnecessary for coding the current syntax element through a bin index(binIdx). Here, the context means information about the occurrenceprobability of the symbol, and the context modeling may mean a processof estimating the probability of the bin necessary for the binaryarithmetic coding by receiving the bin, which is the binarizationresult.

The context modeling unit 420 may provide (accurate) probabilityestimation necessary for achieving enhanced coding efficiency.Therefore, different context models may be used for different binarysymbols and the probability of the context model may be updated based onthe values of the previously coded binary symbol. At this time, thevalues of the previously coded binary symbol are stored in the memory460, and the context modeling unit 420 may use the values of thepreviously coded binary symbol from the memory 460.

The binary symbols having a similar distribution may share the samecontext model. The context model for each binary symbol may use at leastone of the syntax information of the bin, the bin index (binIdx)representing the location of the bin in the bin string, and theprobability of the bin included in the neighbor block of the block inwhich the bin is included, for estimating the probability.

The binary arithmetic encoding unit 430 includes the regular binaryencoding unit 440 and the bypass binary encoding unit 450, and mayperform the entropy encoding for the outputted string and output thecompressed data bits.

The regular binary encoding unit 440 may perform the arithmetic codingbased on a recursive interval division.

First, a section (or range) having an initial value of 0 or 1 may besplit into two lower sections based on the probability of the binarysymbol. The encoded bits may provide an offset selecting one of twolower sections representing a value of the decoded binary symbol whenbeing transformed into the binary fraction.

After the binary symbol of the decoded mode, the section may be updatedto make the selected lower section the same, and the section splitprocess itself may be repeated. The section and the offset have limitedbit precision, and therefore, renormalization may be required to preventoverflow every time the section reduces to a specific value or less. Therenormalization may occur after each binary symbol is decoded.

The bypass binary encoding unit 450 performs the encoding without thecontext model, and may perform the coding by fixing the probability ofthe currently coded bin as 0.5. This may be used when it is difficult todetermine the probability of the syntax or the high-speed coding isrequired.

FIG. 5 is a block diagram illustrating a configuration of an entropydecoder according to the exemplary embodiment.

FIG. 5 illustrates a schematic block diagram of the entropy decoder towhich the context-based adaptive binary arithmetic coding (CABAC) isapplied, as the exemplary embodiment to which the present disclosure isapplied.

An entropy decoder 500 according to the exemplary embodiment may includea context modeling unit 510, a binary arithmetic decoding unit 520, amemory 550, and an inverse binarization unit 560, and the binaryarithmetic decoding unit 520 may include a regular binary decoding unit530 and a bypass binary decoding unit 540.

The entropy decoder 500 may receive a bitstream and confirm a bypassflag therefrom. Here, the bypass flag represents whether the bitstreamis in a bypass mode, and the bypass mode may mean that the coding isperformed by fixing the probability of the currently coded bin as 0.5without using the context model.

If the bitstream is not in the bypass mode according to the bypass flag,the regular binary decoding unit 530 may perform the binary arithmeticdecoding according to a regular mode.

At this time, the context modeling unit 510 may select the probabilityinformation necessary for decoding a current bitstream from the memory550 to transmit the probability information to the regular binarydecoding unit 530.

Meanwhile, if the bitstream is in the bypass mode according to thebypass flag, the bypass binary decoding unit 540 may perform the binaryarithmetic decoding according to the bypass mode.

The inverse binarization unit 560 may receive the bin of the binary formdecoded by the binary arithmetic decoding unit 520 and transform the bininto a syntax element value in an integer form to output the transformedsyntax element value.

FIG. 6 is a flowchart illustrating an operation of an entropy encoderaccording to the exemplary embodiment.

The entropy encoder of the encoding apparatus according to the exemplaryembodiment may perform binarization for the syntax element (S610).

The entropy encoder according to the exemplary embodiment may determinewhether to perform the binary arithmetic coding according to the regularmode or to perform the binary arithmetic coding according to the bypassmode (S620). For example, the entropy encoder may confirm whether thebitstream is in the regular mode or the bypass mode based on the bypassflag, and when the bypass flag is 1, it may represent the bypass mode,and when the bypass flag is 0, it may represent the regular mode.

If it is determined that the binary arithmetic coding is to be performedaccording to the regular mode, the entropy encoder may select theprobability model (S630), and perform the binary arithmetic encodingbased on the probability model (S640). Further, the entropy encoder mayupdate the probability model (S650), and select a suitable probabilitymodel again based on the probability model updated in the step S630.

If it is determined that the binary arithmetic coding is to be performedaccording to the bypass mode, the entropy encoder may perform the binaryarithmetic encoding based on the probability of 0.5 (S660).

FIG. 7 is a flowchart illustrating an operation of the entropy decoderaccording to the exemplary embodiment.

The entropy decoder of the decoding apparatus according to the exemplaryembodiment may receive the bitstream (S710).

The entropy decoder according to the exemplary embodiment may confirmwhether the bitstream is in the regular mode or the bypass mode byextracting the bypass flag from the bitstream (S720). Here, the bypassflag may be previously determined according to the kind of syntax.

If the bypass flag represents the regular mode, the entropy decoder mayselect the probability model (S730), and perform the binary arithmeticdecoding based on the selected probability model (S740). Further, theentropy decoder may update the probability model (S750), and select thesuitable probability model again based on the probability model updatedin the step S730.

Meanwhile, if the bypass flag represents the bypass mode, the entropydecoder may perform the binary arithmetic decoding based on theprobability of 0.5 (S760).

The entropy decoder may perform inverse binarization for the decoded binstring (S770). For example, the entropy decoder may receive the decodedbin in the binary form and transform the bin into the syntax elementvalue in the integer form to output the syntax element value.

The exemplary embodiment of the present disclosure proposes a method fordetermining the context model for the CABAC entropy encoding. Asdescribed above, the split_bt_flag may represent whether the currentblock is split into the BT or split into the TT. The syntax elementsplit_bt_flag may be referred to as mtt_split_cu_binary_flag or thelike. The context model for the syntax element split_bt_flag may bedetermined according to the number of BTs and TTs which may be splitfrom the current block. Equation 1 below expresses a value of thecontext index (CtxIdx) according to block split availabilityinformation.

CtxIdx=(availableBTHOR && availableBTVER)*2+(availableTTHOR &&availableTTVER)  Equation 1

In the present specification, the “block split availability information”may represent a block split condition. The block split availabilityinformation may include the binary tree (BT) horizontal split availableflag information representing whether the current block may behorizontally split based on the BT structure, the ternary tree (TT)horizontal split available flag information representing whether thecurrent block may be horizontally split based on the TT structure, theBT vertical split available flag information representing whether thecurrent block may be vertically split based on the BT structure, and theTT vertical split available flag information representing whether thecurrent block may be vertically split based on the TT structure.According to the exemplary embodiment, the block split availabilityinformation may be derived based on at least one of block sizeinformation, multi-type tree depth information, and the maximummulti-tree size information.

In the Equation 1, the availableBTHOR may represent the BT horizontalsplit available flag information, the availableBTVER may represent theBT vertical split available flag information, the availableTTHOR mayrepresent the TT horizontal split available flag information, and theavailableTTVER may represent the TT vertical split available flaginformation. A range of a value of the CtxIdx may be 0 or more and 3 orless, for example.

The BT horizontal split available flag information may be indicated byvarious names such as available_BT_HOR and allow SplitBTHor as well asavailableBTHOR. Likewise, the BT vertical split available flaginformation may be indicated by available_BT_VER, allowSplitBTVer, orthe like, the TT horizontal split available flag information may beindicated by available_TT_HOR, allowSplitTTHor, or the like, and the TTvertical split available flag information may be indicated byavailable_TT_VER, allowSplitTTVer, or the like.

Table 5 below expresses an example of the values of the CtxIdx accordingto the values of the availableBTHOR, the availableBTVER, theavailableTTHOR, and the availableTTVER.

TABLE 5 availableBTHOR availableBTVER availableTTHOR availableTTVERCtxIdx 0 0 0 0 N/A 0 0 0 1 N/A 0 0 1 0 N/A 0 0 1 1 N/A 0 1 0 0 N/A 0 1 01 0 0 1 1 0 0 0 1 1 1 1 1 0 0 0 N/A 1 0 0 1 0 1 0 1 0 0 1 0 1 1 1 1 1 00 N/A 1 1 0 1 2 1 1 1 0 2 1 1 1 1 3

When the value of each of the availableBTHOR, the availableBTVER, theavailableTTHOR, and the availableTTVER is 0, it represents that thesplit according to each split structure is not available, and when eachvalue is 1, it represents that the split according to the splitstructure is available. If the value of the CtxIdx is N/A, it mayrepresent that it is unnecessary to determine the value of the CtxIdxbecause the signaling of the split_bt_flag is unnecessary.

According to the exemplary embodiment, four context models may be usedfor encoding the split_bt_flag. Table 6 below expresses an example ofthe initial values (InitValue) of the CABAC according to the values ofthe CtxIdx.

TABLE 6 CtxIdx InitValue 0 152 1 153 2 154 3 152

As another example, when the value of the CtxIdx is 0 or 3, the sameinitial values of the CABAC may be used, and when having the similarstatistic property, three context models may be used for encoding thesplit_bt_flag. That is, when the value of the CtxIdx is 3, the value ofthe CtxIdx may be replaced with 0. Table 7 below expresses an example ofthe initial values of the CABAC according to the values of the CtxIdx,when using three context models.

TABLE 7 CtxIdx InitValue 0 152 1 153 2 154

Another exemplary embodiment of the present disclosure proposes anothermethod for determining the context model for the CABAC entropy encoding.As an example, the syntax element split_bt_flag may be first signaled todetermine whether it is the BT split or the TT split, and then thesyntax element split_dir may be signaled to determine whether it is thehorizontal split or the vertical split. The syntax element split_dir mayalso be referred to as mtt_split_cu_vertical_flag in some cases. Asanother example, the syntax element split_dir may be first signaled todetermine whether it is the horizontal split or the vertical split, andthen the syntax element split_bt_flag may be signaled to determinewhether it is the BT split or the TT split.

At this time, the context model for the syntax element split_dir may bedetermined according to the number of horizontal splits and verticalsplits which may be split from the current block. Equation 2 belowexpresses a value of the context index (CtxIdx) according to the blocksplit availability information.

CtxIdx=(availableBTHOR && availableTTHOR)*2+(availableBTVER &&availableTTVER)  Equation 2

Table 8 below expresses an example of the values of the CtxIdx accordingto the values of the availableBTHOR, the availableBTVER, theavailableTTHOR, and the availableTTVER.

TABLE 8 availableBTHOR availableTTHOR availableBTVER availableTTVERCtxIdx 0 0 0 0 N/A 0 0 0 1 N/A 0 0 1 0 N/A 0 0 1 1 N/A 0 1 0 0 N/A 0 1 01 0 0 1 1 0 0 0 1 1 1 1 1 0 0 0 N/A 1 0 0 1 0 1 0 1 0 0 1 0 1 1 1 1 1 00 N/A 1 1 0 1 2 1 1 1 0 2 1 1 1 1 3

In the Table 8, the case where the value of the CtxIdx is N/A mayrepresent the case where it is unnecessary to determine the value of theCtxIdx because the signaling of the split_dir_flag is unnecessary.

According to the exemplary embodiment, referring to the Table 8, if thevalue of the availableBTHOR and the value of the availableTTHOR are 0 orthe values of the availableBTVER and the availableTTVER are 0, it may beconfirmed that the CtxIdx for the split_dir is not derived.

According to the exemplary embodiment, referring to the Table 8, when avalue of the (availableBTHOR∥availableTTHOR) &&(availableBTVER∥availableTTVER) is 0, it may be confirmed that theCtxIdx for the split_dir is not derived.

According to the exemplary embodiment, four context models may be usedfor encoding the split_dir_flag. Table 9 below expresses an example ofthe initial values (InitValue) of the CABAC according to the values ofthe CtxIdx.

TABLE 9 CtxIdx InitValue 0 154 1 155 2 153 3 154

As another example, when the value of the CtxIdx is 0 or 3, the sameinitial values of the CABAC are used, and when having the similarstatistic property, three context models may be used for encoding thesplit_bt_flag. That is, when the value of the CtxIdx is 3, the value ofthe CtxIdx may be replaced with 0. Table 10 below expresses an exampleof the initial values of the CABAC according to the values of theCtxIdx, in the case of using three context models.

TABLE 10 CtxIdx InitValue 0 154 1 155 2 153

The exemplary embodiment may determine the block split according to thetransform size. An available transform size for the currently decodedimage or picture may be transmitted from the unit of a sequenceparameter set (SPS), a picture parameter set (PPS), a slice header (SH),a header of a network abstract layer (NAL) unit, or the like. For theavailable transform size, the corresponding transform size may also betransmitted one by one, and the corresponding index may also betransmitted based on a predefined transform set. The decoder terminalmay receive and decode the index to induce the available transform size.

When the available transform size is determined, the size of theavailable block split may be determined. That is, the size of theavailable block split is defined for the currently decoded image andpicture. The size of the available block split may be determined by thecase where the transform is available. Therefore, the split of the blockin which the transform does not exist may not be allowed.

The size of the available block split is determined for the image andpicture, and then it may be determined whether the block may be split inthe block unit, that is, a CU or PU unit. The kind of available blocksplit is determined based on the size of the current block. That is, theBT or TT available candidates may be configured horizontally andvertically. Alternatively, whether the splits of the ¼, ¾ rates, thesplits of the ⅜, ⅝ rates, the splits of the ⅓, ⅔ rates, the splits ofthe ⅕, ⅘ rates, or the like in addition to the split of the ½ rate areavailable may be determined.

The split available block candidates are configured, and then a finalblock split may be determined by the block split syntax transmitted tothe decoder. If one available block candidate exists or no availableblock candidate exists, the corresponding syntax may not be transmitted.

FIG. 8 is a flowchart illustrating an operation of the encodingapparatus according to the exemplary embodiment, and FIG. 9 is a blockdiagram illustrating a configuration of the encoding apparatus accordingto the exemplary embodiment.

The encoding apparatus illustrated in FIGS. 8 and 9 may perform theoperation corresponding to that of the decoding apparatus to bedescribed later illustrated in FIGS. 10 and 11. Therefore, the contentsto be described later with reference to FIGS. 10 and 11 may also beequally applied to the encoding apparatus illustrated in FIGS. 8 and 9.

Each step illustrated in FIG. 8 may be performed by the encodingapparatus 100 illustrated in FIG. 1. More specifically, S800 to S830 maybe performed by the entropy encoder 130 illustrated in FIG. 1. Further,operations according to the S800 to S830 are performed based on some ofthe contents described above with reference to FIGS. 3 to 7. Therefore,the description of specific contents overlapping the contents describedabove with reference to FIGS. 3 to 7 will be omitted or simplified.

As illustrated in FIG. 9, the encoding apparatus according to theexemplary embodiment may include a picture partitioning module 105 andthe entropy encoder 130. However, in some cases, all of the componentsillustrated in FIG. 9 may not be essential components of the encodingapparatus, and the encoding apparatus may be implemented by a largernumber or a fewer number of components than the components illustratedin FIG. 9.

In the encoding apparatus according to the exemplary embodiment, each ofthe picture partitioning module 105 and the entropy encoder 130 may beimplemented by a separate chip, or two or more components may also beimplemented by a single chip.

The encoding apparatus according to the exemplary embodiment may derivea plurality of different coding units by splitting the current blockwithin the current picture. More specifically, the picture partitioningmodule 105 of the encoding apparatus may derive a plurality of differentcoding units by splitting the current block within the current picture.

The encoding apparatus according to the exemplary embodiment may derivethe split flag syntax element representing the split structure of thecurrent block (S800). More specifically, the entropy encoder 130 of theencoding apparatus may derive the split flag syntax element representingthe split structure of the current block.

According to the exemplary embodiment, the split flag syntax element mayinclude the directional split flag representing whether the currentblock is vertically split and the split syntax element may include theBT split flag representing whether the current block is split based onthe BT structure. The directional split flag may be, for example,indicated by the split_dir, and the BT split flag may be, for example,indicated by the split_bt_flag.

The encoding apparatus according to the exemplary embodiment may derivethe context index for the split flag syntax element based on the splitavailability information of the current block (S810). More specifically,the entropy encoder 130 of the encoding apparatus may derive the contextindex for the split flag syntax element based on the split availabilityinformation of the current block.

According to the exemplary embodiment, the split availabilityinformation may include the BT horizontal split available flaginformation representing whether the current block may be horizontallysplit based on the BT structure, the TT horizontal split available flaginformation representing whether the current block may be horizontallysplit based on the TT structure, the BT vertical split available flaginformation representing whether the current block may be verticallysplit based on the BT structure, and the TT vertical split availableflag information representing whether the current block may bevertically split based on the TT structure.

According to the exemplary embodiment, the BT horizontal split availableflag information may be indicated by the availableBTHOR, the TThorizontal split available flag information may be indicated by theavailableTTHOR, the BT vertical split available flag information may beindicated by the availableBTVER, and the TT vertical split availableflag information may be indicated by the availableTTVER.

According to the exemplary embodiment, the context index for thedirectional split flag may be derived based on the BT horizontal splitavailable flag information, the TT horizontal split available flaginformation, the BT vertical split available flag information, and theTT vertical split available flag information.

According to the exemplary embodiment, whether the context index for thedirectional split flag is derived may be determined based on the BThorizontal split available flag information, the TT horizontal splitavailable flag information, the BT vertical split available flaginformation, and the TT vertical split available flag information.

According to the exemplary embodiment, if the value of the BT horizontalsplit available flag information and the value of the TT horizontalsplit available flag information are 0, or the value of the BT verticalsplit available flag information and the value of the TT vertical splitavailable flag information are 0, it is characterized in that thecontext index for the vertical split flag is not derived.

According to the exemplary embodiment, when the value of the(availableBTHOR∥availableTTHOR) && (availableBTVER∥availableTTVER) is 0,the context index for the vertical split flag may not be derived.

According to the exemplary embodiment, the context index for the BTsplit flag may be derived based on the BT horizontal split availableflag information, the TT horizontal split available flag information,the BT vertical split available flag information, and the TT verticalsplit available flag information.

According to the exemplary embodiment, the context index for thedirectional split flag may be derived based on Equation 3 below.

CtxIdx=(availableBTHOR && availableTTHOR)*2+(availableBTVER &&availableTTVER)  Equation 3

According to the exemplary embodiment, the context index for the BTsplit flag may be derived based on Equation 4 below.

CtxIdx=(availableBTHOR && availableBTVER)*2+(availableTTHOR &&availableTTVER)  Equation 4

The encoding apparatus according to the exemplary embodiment maydetermine the context model based on the derived context index (S820).More specifically, the entropy encoder 130 of the encoding apparatus maydetermine the context model based on the derived context index.

The encoding apparatus according to the exemplary embodiment may encodethe value of the split flag syntax element based on the CABAC using thedetermined context model (S830). More specifically, the entropy encoder130 of the encoding apparatus may encode the value of the split flagsyntax element based on the CABAC using the determined context model.

According to the encoding apparatus and the method for operating theencoding apparatus illustrated in FIGS. 8 and 9, the encoding apparatusmay derive the split flag syntax element representing the splitstructure of the current block (S800), derive the context index for thesplit flag syntax element based on the split availability information ofthe current block (S810), determine the context model based on thederived context index (S820), and encode the value of the split flagsyntax element based on the CABAC using the determined context model(S830), and at this time, it may be characterized in that the split flagsyntax element may include the directional split flag representingwhether the current block is vertically split, the split availabilityinformation may include the BT horizontal split available flaginformation representing whether the current block may be verticallysplit based on the BT structure, the TT horizontal split available flaginformation representing whether the current block may be horizontallysplit based on the TT structure, the BT vertical split available flaginformation representing whether the current block may be verticallysplit based on the BT structure, and the TT vertical split availableflag information representing whether the current block may bevertically split based on the TT structure, and the context index forthe directional split flag is derived based on the BT horizontal splitavailable flag information, the TT horizontal split available flaginformation, the BT vertical split available flag information, and theTT vertical split available flag information. That is, as illustrated inFIGS. 8 and 9, when the entropy encoding according to the CABAC isperformed based on the QT structure, the BT structure, and the TTstructure, it is possible to perform the encoding and the decoding moreefficiently using the context model.

FIG. 10 is a flowchart illustrating an operation of the decodingapparatus according to the exemplary embodiment, and FIG. 11 is a blockdiagram illustrating a configuration of the decoding apparatus accordingto the exemplary embodiment.

Each step illustrated in FIG. 10 may be performed by the decodingapparatus 200 illustrated in FIG. 2. More specifically, S1000 to S1030may be performed by the entropy decoder 210 illustrated in FIG. 2, S1040may be performed by the predictor 230 illustrated in FIG. 2, and S1050may be performed by the adder 240 illustrated in FIG. 2. Further,operations according to the S1000 to S1050 are performed based on someof the contents described above with reference to FIGS. 3 to 7.Therefore, the description of the specific contents overlapping thecontents described above with reference to FIGS. 3 to 7 will be omittedor simplified.

As illustrated in FIG. 11, the decoding apparatus according to theexemplary embodiment may include the entropy decoder 210, the predictor230, and the adder 240. However, in some cases, all of the componentsillustrated in FIG. 11 may not be the essential components of thedecoding apparatus, and the decoding apparatus may be implemented by alarger number or a fewer number of components than the componentsillustrated in FIG. 11.

In the decoding apparatus according to the exemplary embodiment, each ofthe entropy decoder 210, the predictor 230, and the adder 240 may beimplemented by a separate chip, or two or more components may also beimplemented by a single chip.

The decoding apparatus according to the exemplary embodiment may derivethe context index for the split flag syntax element based on the splitavailability information of the current block (S1000). Morespecifically, the entropy decoder 210 of the decoding apparatus mayderive the context index for the split flag syntax element based on thesplit availability information of the current block.

According to the exemplary embodiment, the split flag syntax element mayinclude the directional split flag representing whether the currentblock is vertically split and the split syntax element may include theBT split flag representing whether the current block is split based onthe BT structure. The directional split flag may be, for example,indicated by the split_dir, and the BT split flag may be, for example,the split_bt_flag.

According to the exemplary embodiment, the split availabilityinformation may include the BT horizontal split available flaginformation representing whether the current block may be horizontallysplit based on the BT structure, the TT horizontal split available flaginformation representing whether the current block may be horizontallysplit based on the TT structure, the BT vertical split available flaginformation representing whether the current block may be verticallysplit based on the BT structure, and the TT vertical split availableflag information representing whether the current block may bevertically split based on the TT structure.

As an example, the BT horizontal split available flag information may beindicated by the availableBTHOR, the TT horizontal split available flaginformation may be indicated by the availableTTHOR, the BT verticalsplit available flag information may be indicated by the availableBTVER,and the TT vertical split available flag information may be indicated bythe availableTTVER.

According to the exemplary embodiment, the context index for thedirectional split flag may be derived based on the BT horizontal splitavailable flag information, the TT horizontal split available flaginformation, the BT vertical split available flag information, and theTT vertical split available flag information.

According to the exemplary embodiment, whether the context index for thedirectional split flag is derived may be determined based on the BThorizontal split available flag information, the TT horizontal splitavailable flag information, the BT vertical split available flaginformation, and the TT vertical split available flag information.

According to the exemplary embodiment, if the value of the BT horizontalsplit available flag information and the value of the TT horizontalsplit available flag information are 0, or the value of the BT verticalsplit available flag information and the value of the TT vertical splitavailable flag information are 0, the context index for the directionalsplit flag may not be derived.

According to the exemplary embodiment, when the value of the(availableBTHOR∥availableTTHOR) && (availableBTVER∥availableTTVER) is 0,the context index for the directional split flag may not be derived.

According to the exemplary embodiment, the context index for the BTsplit flag may be derived based on the BT horizontal split availableflag information, the TT horizontal split available flag information,the BT vertical split available flag information, and the TT verticalsplit available flag information.

According to the exemplary embodiment, the context index for thedirectional split flag may be derived based on Equation 5 below.

CtxIdx=(availableBTHOR && availableTTHOR)*2+(availableBTVER &&availableTTVER)  Equation 5

According to the exemplary embodiment, the context index for the BTsplit flag may be derived based on Equation 6 below.

CtxIdx=(availableBTHOR && availableBTVER)*2+(availableTTHOR &&availableTTVER)  Equation 6

The decoding apparatus according to the exemplary embodiment maydetermine the context model based on the derived context index (S1010).More specifically, the entropy decoder 210 of the decoding apparatus maydetermine the context model based on the derived context index.

The decoding apparatus according to the exemplary embodiment may decodethe value of the split flag syntax element based on the context-basedadaptive binary arithmetic coding (CABAC) using the determined contextmodel (S1020). More specifically, the entropy decoder 210 of thedecoding apparatus may decode the value of the split flag syntax elementbased on the CABAC using the determined context model.

The decoding apparatus according to the exemplary embodiment may derivethe current coding unit from the current block based on the value of thesplit flag syntax element (S1030). More specifically, the entropydecoder 210 of the decoding apparatus may derive the current coding unitfrom the current block based on the value of the split flag syntaxelement.

The decoding apparatus according to the exemplary embodiment may derivethe predicted block based on the inter-prediction or theintra-prediction for the current coding unit (S1040). More specifically,the predictor 230 of the decoding apparatus may derive the predictedblock based on the inter-prediction or the intra-prediction for thecurrent coding unit.

The decoding apparatus according to the exemplary embodiment maygenerate a reconstructed block based on the predicted block (S1050).More specifically, the adder 240 of the decoding apparatus may generatethe reconstructed block based on the predicted block.

According to the decoding apparatus and the method for operating thedecoding apparatus illustrated in FIGS. 10 and 11, the decodingapparatus may derive the context index for the split flag syntax elementbased on the split availability information of the current block(S1000), determine the context model based on the derived context index(S1010), decode the value of the split flag syntax element based on thecontext-based adaptive binary arithmetic coding (CABAC) using thedetermined context model (S1020), derive the current coding unit fromthe current block based on the value of the split flag syntax element(S1030), derive the predicted block based on the inter-prediction or theintra-prediction for the current coding unit (S1040), and generate thereconstructed block based on the predicted block (S1050), and at thistime, it may be characterized in that the split flag syntax element mayinclude the directional split flag representing whether the currentblock is vertically split, the split availability information mayinclude the BT horizontal split available flag information representingwhether the current block may be horizontally split based on the BTstructure, the TT horizontal split available flag informationrepresenting whether the current block may be horizontally split basedon the TT structure, the BT vertical split available flag informationrepresenting whether the current block may be vertically split based onthe BT structure, and the TT vertical split available flag informationrepresenting whether the current block may be vertically split based onthe TT structure; and the context index for the directional split flagis derived based on the BT horizontal split available flag information,the TT horizontal split available flag information, the BT verticalsplit available flag information, and the TT vertical split availableflag information. That is, as illustrated in FIGS. 10 and 11, when theentropy encoding according to the CABAC is performed based on the QTstructure, the BT structure, and the TT structure, it is possible toperform the encoding and the decoding more efficiently using the contextmodel.

The exemplary embodiments described in the present disclosure may beimplemented and performed by a processor, a microprocessor, acontroller, or a chip. For example, the functional units illustrated ineach drawing may be implemented and performed by a computer, theprocessor, the microprocessor, the controller, or the chip.

Further, the decoder and the encoder to which the exemplary embodimentsof the present disclosure are applied may be included in a multimediabroadcasting transceiver, a mobile communication terminal, a home cinemavideo device, a digital cinema video device, a surveillance camera, avideo chat device, a real-time communication device such as a videocommunication device, a mobile streaming device, a storage medium, acamcorder, a video on demand (VoD) service providing device, an over thetop (OTT) video device, an Internet streaming service providing device,a three-dimensional (3D) video device, an image telephony video device,a medial video device, or the like, and used for processing a videosignal or a data signal. For example. the over the top (OTT) videodevice may include a game console, a Blue-ray player, an Internet accessTV, a home theater system, a smartphone, a tablet PC, a digital videorecorder (DVR), or the like.

Further, the processing method to which the exemplary embodiments of thepresent disclosure are applied may be produced in the form of a programexecuted by a computer, and stored in a computer-readable recordingmedium. Multimedia data having a data structure according to the presentdisclosure may also be stored in a computer-readable recording medium.The computer-readable recording medium includes all kinds of storagedevices and distributed storage devices in which computer-readable dataare stored. The computer-readable recording medium may include, forexample, a Blu-ray Disc (BD), a universal serial bus (USB), a ROM, aPROM, an EPROM, an EEPROM, a RAM, a CD-ROM, a magnetic tape, a floppydisk, and an optical data storage device.

Further, the computer-readable recording medium includes mediaimplemented in the form of a carrier wave (for example, transmissionover the Internet). Further, a bitstream generated by the encodingmethod may be stored in a computer-readable recording medium ortransmitted through a wired or wireless communication network.

Further, the exemplary embodiments of the present disclosure may beembodied as a computer program product by program codes, and the programcodes may be executed on a computer by the exemplary embodiments of thepresent disclosure. The program codes may be stored on acomputer-readable carrier.

FIG. 12 is a contents streaming system structure diagram according tothe exemplary embodiment.

The contents streaming system to which the present disclosure is appliedmay largely include an encoding server, a streaming server, a webserver, a media storage, user equipment, and a multimedia input device.

The encoding server serves to compress, to digital data, the contentsinput from the multimedia input devices, such as a smartphone, a camera,and a camcoder, to generate a bitstream, and to transmit the bitstreamto the streaming server. As another example, if the multimedia inputdevice, such as a smartphone, a camera, or a camcoder, directlygenerates a bitstream, the encoding server may be omitted.

The bitstream may be generated by the encoding method or the bitstreamgeneration method to which the present disclosure is applied, and thestreaming server may temporarily store the bitstream in a process oftransmitting or receiving the bitstream.

The streaming server transmits multimedia data to the user equipmentbased on a user's request through the web server, and the web serverserves as a medium which informs the user of what service there is. Whenthe user requests a desired service to the web server, the web servertransfers the service to the streaming server, and the streaming servertransmits multimedia data to the user. At this time, the contentsstreaming system may include a separate control server, and in thiscase, the control server serves to control instructions/responsesbetween respective equipment in the contents streaming system.

The streaming server may receive contents from the media storage and/orthe encoding server. For example, if the contents are received from theencoding server, the contents may be received in real time. In thiscase, the streaming server may store the bitstream for a predeterminedtime to provide the streaming service smoothly.

Examples of the user equipment may include a mobile phone, a smartphone,a laptop computer, a digital broadcasting terminal, a personal digitalassistant (PDA), a portable multimedia player (PMP), a navigationdevice, a slate PC, a tablet PC, an ultrabook, a wearable device (e.g.,a watch-type terminal (smart watch), a glass-type terminal (smartglass), a head mounted display (HMD)), a digital TV, a desktop computer,a digital signage or the like.

The respective servers in the contents streaming system may be operatedas a distributed server, and in this case, data received by each servermay be processed in distributed manner.

The aforementioned method according to the present disclosure may beembodied in the form of software, and the encoding apparatus and/ordecoding apparatus according to the present disclosure may be includedin an apparatus for performing the image processing, such as a TV, acomputer, a smartphone, a set-top box, or a display device.

Each of the part, the module, or the unit described above may be aprocessor or a hardware part executing consecutive implementationprocesses stored in the memory (or storage unit). Each step described inthe aforementioned exemplary embodiment may be performed by theprocessor or the hardware parts. The respective modules/blocks/unitsdescribed in the aforementioned exemplary embodiment may be operated asthe hardware/the processor. Further, the methods provided by the presentdisclosure may be executed as a code. This code may be written in theprocessor-readable storage medium, and therefore, read by the processorprovided by the apparatus.

In the aforementioned exemplary embodiments, the methods are explainedbased on flowcharts by means of a series of steps or blocks, but thepresent disclosure is not limited to the order of steps, and a certainstep may occur in order or step different from that described above, orconcurrently with that described above. Further, it may be understood bya person having ordinary skill in the art that the steps shown in aflowchart are not exclusive, and that another step may be incorporatedor one or more steps of the flowchart may be removed without affectingthe scope of the present disclosure.

When the exemplary embodiments according to the present disclosure areembodied by software, the aforementioned methods may be embodied bymodules (processes, functions, or the like) performing theaforementioned function. The module may be stored in a memory andexecuted by a processor. The memory may be inside or outside theprocessor and connected to the processor via well-known various means.The processor may include an application-specific integrated circuit(ASIC), other chipset, a logic circuit, and/or a data processing device.The memory may include a read-only memory (ROM), a random access memory(RAM), a flash memory, a memory card, a storage medium, and/or otherstorage device.

1. A picture decoding method performed by a decoding apparatus, themethod comprising: deriving a context index for a split flag syntaxelement based on split availability information of a current block;determining a context model based on the derived context index; decodinga value of the split flag syntax element based on context-based adaptivebinary arithmetic coding (CABAC) by using the determined context model;deriving a current coding unit from the current block based on the valueof the split flag syntax element; deriving a predicted block based oninter-prediction or intra-prediction for the current coding unit; andgenerating a reconstructed block based on the predicted block, whereinthe split flag syntax element comprises: a directional split flagrepresenting whether the current block is to be vertically split,wherein the split availability information comprises: a binary tree (BT)horizontal split available flag information related to whether thecurrent block is allowed to be horizontally split based on a BTstructure, a ternary tree (TT) horizontal split available flaginformation related to whether the current block is allowed to behorizontally split based on a TT structure, a BT vertical splitavailable flag information related to whether the current block isallowed to be vertically split based on the BT structure, and a TTvertical split available flag information related to whether the currentblock is allowed to be vertically split based on the TT structure, andwherein the context index for the directional split flag is derivedbased on the BT horizontal split available flag information, the TThorizontal split available flag information, the BT vertical splitavailable flag information, and the TT vertical split available flaginformation.
 2. The picture decoding method of claim 1, wherein whetherthe context index for the directional split flag is derived isdetermined based on the BT horizontal split available flag information,the TT horizontal split available flag information, the BT verticalsplit available flag information, and the TT vertical split availableflag information.
 3. The picture decoding method of claim 2, wherein ifa value of the BT horizontal split available flag information and avalue of the TT horizontal split available flag information are 0, or avalue of the BT vertical split available flag information and a value ofthe TT vertical split available flag information are 0, the contextindex for the directional split flag is not derived.
 4. The picturedecoding method of claim 3, wherein when a value of(availableBTHOR∥availableTTHOR) && (availableBTVER∥availableTTVER) is 0,the context index for the directional split flag is not derived, wherethe availableBTHOR represents the BT horizontal split available flaginformation, the availableTTHOR represents the TT horizontal splitavailable flag information, the availableBTVER represents the BTvertical split available flag information, and the availableTTVERrepresents the TT vertical split available flag information.
 5. Thepicture decoding method of claim 1, wherein the split syntax elementfurther comprises: a BT split flag representing whether the currentblock is split based on the BT structure, and wherein the context indexfor the BT split flag is derived based on the BT horizontal splitavailable flag information, the TT horizontal split available flaginformation, the BT vertical split available flag information, and theTT vertical split available flag information.
 6. The picture decodingmethod of claim 1, wherein the context index for the directional splitflag is derived based on Equation below,CtxIdx=(availableBTHOR && availableTTHOR)*2+(availableBTVER &&availableTTVER), in the Equation, the availableBTHOR represents the BThorizontal split available flag information, the availableTTHORrepresents the TT horizontal split available flag information, theavailableBTVER represents the BT vertical split available flaginformation, and the availableTTVER represents the TT vertical splitavailable flag information.
 7. The picture decoding method of claim 5,wherein the context index for the BT split flag is derived based onEquation below,CtxIdx=(availableBTHOR && availableBTVER)*2+(availableTTHOR &&availableTTVER), in the Equation, the availableBTHOR represents the BThorizontal split available flag information, the availableTTHORrepresents the TT horizontal split available flag information, theavailableBTVER represents the BT vertical split available flaginformation, and the availableTTVER represents the TT vertical splitavailable flag information.
 8. A picture encoding method performed by anencoding apparatus, the method comprising: deriving a split flag syntaxelement representing a split structure of a current block; deriving acontext index for the split flag syntax element based on splitavailability information of the current block; determining a contextmodel based on the derived context index; and encoding a value of thesplit flag syntax element based on CABAC using the determined contextmodel, wherein the split flag syntax element comprises: a directionalsplit flag representing whether the current block is vertically split,wherein the split availability information comprises: a BT horizontalsplit available flag information related to whether the current block isallowed to be horizontally split based on a BT structure, a TThorizontal split available flag information related to whether thecurrent block is allowed to be horizontally split based on a TTstructure, a BT vertical split available flag information related towhether the current block is allowed to be vertically split based on theBT structure, and a TT vertical split available flag information relatedto whether the current block is allowed to be vertically split based onthe TT structure, and wherein the context index for the directionalsplit flag is derived based on the BT horizontal split available flaginformation, the TT horizontal split available flag information, the BTvertical split available flag information, and the TT vertical splitavailable flag information.
 9. The picture encoding method of claim 8,wherein whether the context index for the directional split flag isderived is determined based on the BT horizontal split available flaginformation, the TT horizontal split available flag information, the BTvertical split available flag information, and the TT vertical splitavailable flag information.
 10. The picture encoding method of claim 9,wherein if a value of the BT horizontal split available flag informationand a value of the TT horizontal split available flag information are 0,or a value of the BT vertical split available flag information and avalue of the TT vertical split available flag information are 0, thecontext index for the directional split flag is not derived.
 11. Thepicture encoding method of claim 10, wherein when a value of(availableBTHOR∥availableTTHOR) && (availableBTVER∥availableTTVER) is 0,the context index for the directional split flag is not derived, wherethe availableBTHOR represents the BT horizontal split available flaginformation, the availableTTHOR represents the TT horizontal splitavailable flag information, the availableBTVER represents the BTvertical split available flag information, and the availableTTVERrepresents the TT vertical split available flag information.
 12. Thepicture encoding method of claim 8, wherein the split syntax elementfurther comprises: a BT split flag representing whether the currentblock is split based on the BT structure, and wherein the context indexfor the BT split flag is derived based on the BT horizontal splitavailable flag information, the TT horizontal split available flaginformation, the BT vertical split available flag information, and theTT vertical split available flag information.
 13. The picture encodingmethod of claim 8, wherein the context index for the directional splitflag is derived based on Equation below,CtxIdx=(availableBTHOR && availableTTHOR)*2+(availableBTVER &&availableTTVER), in the Equation, the availableBTHOR represents the BThorizontal split available flag information, the availableTTHORrepresents the TT horizontal split available flag information, theavailableBTVER represents the BT vertical split available flaginformation, and the availableTTVER represents the TT vertical splitavailable flag information.
 14. The picture encoding method of claim 12,wherein the context index for the BT split flag is derived based onEquation below,CtxIdx=(availableBTHOR && availableBTVER)*2+(availableTTHOR &&availableTTVER), in the Equation, the availableBTHOR represents the BThorizontal split available flag information, the availableTTHORrepresents the TT horizontal split available flag information, theavailableBTVER represents the BT vertical split available flaginformation, and the availableTTVER represents the TT vertical splitavailable flag information.
 15. (canceled)
 16. A computer-readablestorage medium storing coded information causing a decoding apparatus toperform a picture decoding method, the method comprising: deriving acontext index for a split flag syntax element based on splitavailability information of a current block; determining a context modelbased on the derived context index; decoding a value of the split flagsyntax element based on context-based adaptive binary arithmetic coding(CABAC) by using the determined context model; deriving a current codingunit from the current block based on the value of the split flag syntaxelement; deriving a predicted block based on inter-prediction orintra-prediction for the current coding unit; and generating areconstructed block based on the predicted block, wherein the split flagsyntax element comprises: a directional split flag representing whetherthe current block is to be vertically split, wherein the splitavailability information comprises: a binary tree (BT) horizontal splitavailable flag information related to whether the current block isallowed to be horizontally split based on a BT structure, a ternary tree(TT) horizontal split available flag information related to whether thecurrent block is allowed to be horizontally split based on a TTstructure, a BT vertical split available flag information related towhether the current block is allowed to be vertically split based on theBT structure, and a TT vertical split available flag information relatedto whether the current block is allowed to be vertically split based onthe TT structure, and wherein the context index for the directionalsplit flag is derived based on the BT horizontal split available flaginformation, the TT horizontal split available flag information, the BTvertical split available flag information, and the TT vertical splitavailable flag information.